The present invention relates to a method of cleaning plugged gravel packs, gravel pack screens and perforation tunnels in a wellbore. More particularly, it relates to a method for cleaning and/or removing plugging materials from a gravel pack completion without damaging the gravel pack material.
Over time, most gravel packs will slowly lose permeability due to the reduction in pore space of the pack. This reduction in pore space can be caused in two ways. First, a scale can precipitate out of the well""s produced fluids. In addition, fines can migrate out of the formation and be trapped in the gravel pack. The pore spaces of the gravel pack become plugged with these precipitates or formation fines. These factors lead to an overall reduction in permeability, resulting in lower production rates.
The plugging medium can potentially be removed from the gravel pack, by dissolving the plugging materials with chemicals or treatment fluids. However the insoluble plugging materials must be removed mechanically.
The present invention applies both chemical and mechanical techniques to clean a dirty, plugged gravel pack. It should be used whenever a gravel pack, screen and/or perforation tunnels exhibit signs of losing permeability due to plugging. The present invention can be used to remove soluble and insoluble fines, precipitates, scales and asphaltenes that can severely restrict the permeability of a gravel pack. Thus, the present invention satisfies a long felt need for a process capable of cleaning plugged gravel packs by removal of soluble and insoluble fines without damaging the gravel pack.
According to the preferred embodiment of the invention, treatment fluids are accurately placed through a gravel pack screen to treat a specific region of a gravel pack, its perforation tunnels, the pack/formation interface and the formation. Two preferred treatments include unplugging the pack by removing and/or dissolving fines and precipitates and placing water control chemicals.
The treatment fluid is uniformly placed behind a screen into a sand or gravel pack by generating a tangential vortex and a localized yet fluctuating pressure gradient in the pack. A tangential vortex is a circulating current spinning about an axis substantially tangential to the wellbore. The tangential vortex directs at least a portion of the return flow of the treatment fluid through the screen and up the gravel pack annulus before entering back through the screen. The efficiency of placing the treatment fluid is increased because the treatment fluid is returned to the surface by way of the gravel pack annulus. The fluctuating pressure gradient drives radial fluid flow through the pack. The fluctuating pressure gradient is achieved by the controlled rotation of a jetting nozzle operating at a flow rate sufficient to generate an impact pressure at the screen proppant interface, yet below a pre-determined critical damage threshold pressure. As use herein, impact pressure shall mean the stagnation pressure of the jet on the surface it impacts. The critical damage threshold pressure shall be understood to mean the pressure at which the impact pressure and the length of time the pressure is applied, is great enough to break more than a small percentage of the proppant particles in the gravel pack. For example, API Recommended Practice 58, permits a maximum of 2% fines for gravel pack proppants in a sieve analysis.
The fluctuating pressure gradient causes the proppant to oscillate and thereby creating relative motion between the particles. This relative motion, not only increases the rate at which treatment fluid can invade a pack but increases the rate at which particles can be mobilized into the flow stream to be transported out of the pack. The oscillating of the proppant particles reduces the friction between the fines and proppant. The forces created by the viscous drag of the fluid on the fine particle can more easily remove it from the pack.
In addition, the energy level of the oscillating pressure at higher impact pressures is enough to abrade deposits off the surface of the proppant particle. The higher strength of man-made proppants can accommodate these high energy levels. Therefore, there are two mechanisms at play to remove fines and precipitates from the pack, first to dissolve the soluble fine particle by chemical means, the other is to abrade the precipitate off the proppant, reduce friction between the particles thereby increasing the rate the particles will mobilize into the flow stream, along with other non-soluble fines and transport them out of the pack.
Rotational motion of the nozzle creates pressure pulsing, which has also proven to be an effective way to clean unwanted deposits (e.g., scales, waxes and asphaltenes) off the inside diameter of the screen face.
A method of uniformly placing a treatment fluid behind a screen in a gravel pack in a wellbore according to one embodiment comprises the steps of delivering a pressure pulsating jet of treatment fluid through a jet nozzle against the screen, and creating a tangential vortex beneath the jet nozzle with the treatment fluid wherein at least a portion of the treatment fluid is directed through the screen and into the gravel pack before returning to the surface. The jet nozzle may be oriented to have an axial downward component to the jet direction. An annular region of slurry with low proppant concentration is created behind the screen wherein the flow rate of the treatment fluid in the upwards direction in the annular region is maintained above the threshold transport velocity to suspend the proppants in the annular region.
A method of cleaning a gravel pack in a wellbore according to another embodiment comprises the steps of positioning a pressure pulsating jet inside a gravel pack screen, delivering a pressure pulsating treatment fluid into the gravel pack through the gravel pack screen with a pressure pulsating jet, dissolving soluble plugging materials in the gravel pack with the treatment fluid, and moving insoluble plugging materials through the gravel pack and circulating the insoluble plugging materials out of the wellbore. The method further comprises displacing the treatment fluid with a displacement fluid with the pressure pulsating jet. In another embodiment of the invention, a method of washing a plugged or partially plugged gravel pack and wellbore comprises the steps of delivering a pressure pulsating treatment fluid into the gravel pack, dissolving soluble fines located in the interstitial pore spaces of the gravel pack with the treatment fluid and reducing the pressure drop as a fluid flows into and through the plugged or partially plugged gravel pack by oscillating the fines contained in the gravel pack with the pulsating fluid. The method further comprises oscillating insoluble and yet undissolved fines located in the interstitial pore spaces of the gravel pack with the pressure pulsating treatment fluid until the insoluble fines move through the gravel pack, and circulating the insoluble fines out of the wellbore. Full coverage of the gravel pack is provided by controlling the flow rate, rate of penetration and impact pressure. Damage is prevented by controlling the impact pressure and flow rate to correspond to the specific gravel pack design. The reaction time between the treatment fluids and the fines and gravel particles may be controlled to prevent damage to the gravel particles. Thus, one is able to pump treatment fluids that will react with both the plugging fines and gravel particles, but because the surface area to volume ratio of the fines is much higher, the fines can be substantially dissolved without damaging the gravel particles.
According to another embodiment of the invention, a method of cleaning a gravel pack in a wellbore is provided comprising the steps of positioning a pressure pulsating jet inside the gravel pack, delivering a pressure pulsating treatment fluid into the gravel pack with the pressure pulsating jet and creating a radial pressure gradient within the gravel pack as the pressure pulsating jet is moved through the gravel pack.
One aspect of the invention is directed to a method of washing a gravel pack in a wellbore comprising the steps of delivering a pressure pulsating treatment fluid into the gravel pack with a pressure pulsating jet and a tangential vortex and dissolving soluble plugging materials in the gravel pack with the treatment fluid.
Another aspect of the invention is directed to a method of washing a gravel pack in a wellbore comprising the steps of delivering a pressure pulsating fluid into the gravel pack with a pressure pulsating jet and a tangential vortex, moving insoluble plugging materials through the gravel pack with the fluid and circulating the insoluble plugging materials out of the wellbore.
The pressure pulsing of the present invention is an improvement over prior jetting systems. The pressure pulsing vibrates plugging materials in the gravel pack. This oscillating movement and/or vibration leads to greater efficiency in delivering treatment fluids deeper and more completely through a gravel pack and into the perforation tunnels. The appropriate impact pressures utilized by the present invention provide sufficient energy to oscillate the fines yet not damage the gravel pack. Thus, production may be improved by dissolving soluble fines and removing insoluble fines from the pore spaces of the pack. Additional objects, features and advantages will be apparent in the written description that follows.